For example, a nuclear power plant that includes a pressurized water reactor (PWR) uses light water as a nuclear reactor coolant and a neutron moderator while keeping the light water as high-temperature and high-pressure water which is not boiled throughout a reactor core, sends the high-temperature and high-pressure water to a vapor generator so as to generate a vapor by a heat exchange operation, and sends the vapor to a turbine generator so as to generate electric power.
In such a nuclear power plant, there is a need to periodically inspect various structures of the pressurized water reactor in order to ensure sufficient safety or reliability. Then, when a problem is found after various inspections, a necessary portion involved with the problem is repaired. For example, in the pressurized water reactor, a nuclear reactor vessel body is provided with a plurality of instrumentation nozzles penetrating a lower end plate. Further, each of the instrumentation nozzles is formed so that an in-core instrumentation guide pipe is fixed to the upper end thereof inside the reactor and a conduit tube is connected to the lower end thereof outside the reactor. Then, a neutron flux detector capable of measuring a neutron flux is insertable from the instrumentation nozzle to a reactor core (a fuel assembly) through the in-core instrumentation guide pipe by using the conduit tube.
The instrumentation nozzle is formed in a manner such that an in-core instrumentation cylinder formed of a nickel base alloy is fitted into an attachment hole of a nuclear reactor vessel body formed of low-alloy steel and is welded by a nickel base alloy. For that reason, there is a possibility that a stress corrosion crack may occur in the in-core instrumentation cylinder due to the long-term use. Thus, when the stress corrosion crack occurs, there is a need to repair the instrumentation nozzle. A nozzle repair method of the related art is disclosed in, for example, Patent Literature 1 below. The nozzle repair method disclosed in Patent Literature 1 includes forming a buttered-grooving portion by excavating an inner surface of a vessel in a substantially cylindrical shape in an area including a J-beveling portion, inserting a plug including a plug body portion with an inner end surface forming an extension portion of an outer surface of the buttered-grooving portion and a protrusion protruding from the inner end surface and having the same axis as the buttered-grooving portion into a nozzle hole so that the inner end surface substantially matches the outer surface of the buttered-grooving portion, forming a buttered-welding portion by buttered welding the buttered-grooving portion, forming a J-beveling portion in the buttered-welding portion, and inserting and welding the nozzle.